This invention relates to the field of high temperature composites made by chemical vapor infiltration and deposition of a binding matrix within a porous structure. More particularly the invention relates to a method of measuring pressure within a CVI/CVD furnace during a CVI/CVD process, as well as an apparatus to measure pressure within a CVI/CVD furnace.
Chemical vapor infiltration and deposition is a well-known process for depositing a binding matrix within a porous structure. The term chemical vapor deposition, xe2x80x9cCVDxe2x80x9d generally implies only the deposition of a surface coating. However, the term has also been used to describe not only the deposition but also the infiltration of a matrix within a porous structure. Therefore, for purposes of this application, the term chemical vapor infiltration, xe2x80x9cCVI/CVDxe2x80x9d is intended to refer to not only the infiltration but also the deposition of a binding matrix within a porous structure. This technique is particularly suitable for fabricating high temperature structural composites by depositing a carbonaceous or ceramic matrix within a carbonaceous or ceramic porous structure resulting in very useful structures such as carbon/carbon aircraft brake disks, and ceramic combustor or turbine components.
In general, in the CVI/CVD process for carbon deposition, an open or porous structure is exposed to a gas under conditions that decompose or crack the gas into carbon very slowly. The carbon is deposited into a porous structure to form a composite material comprising the porous structure with a matrix filler within the porous structure. In conventional CVI processes, the outside of the porous structure densifies more quickly than the interior of the structure. This densification decreases the porosity of the surface and prohibits the hydrocarbon gas from entering the porous structure. Therefore, the process must be interrupted and the porosity restored by machining the porous structure to remove the occluded surface.
The generally known CVI/CVD processes may be classified into four general categories: isothermal, thermal gradient, pressure gradient, and pulsed flow. See W. V. Kotlensky, Deposition of Pyrolytic Carbon in Porous Solids, 8 Chemistry and Physics of Carbon, 173, 190-203 (1973); W. J. Lackey, Review, Status, and Future of the Chemical Vapor Infiltration Process for Fabrication of Fiber-Reinforced Ceramic Composites, Ceram. Eng. Sci. Proc. 10[7-8] 577, 577-81 (1989). In an isothermal CVI/CVD process, a reactant gas passes around a heated porous structure at absolute pressures as low as a few millitorr. The gas diffuses into the porous structure driven by concentration gradients and cracks to deposit a binding matrix. This process is also known as xe2x80x9cconventionalxe2x80x9d CVI/CVD. Although the process is coined xe2x80x9cisothermalxe2x80x9d due to the fact that the porous structure is heated to a more or less uniform temperature, this is actually a misnomer. Some variations in temperature within the porous structure are inevitable due to uneven heating (essentially unavoidable in most furnaces), cooling of some portions due to reactant gas flow, and heating or cooling of other portions due to heat of reaction effects. Hence, xe2x80x9cisothermalxe2x80x9d means that there is no attempt to induce a thermal gradient that preferentially affects deposition of a binding matrix onto the porous structure. This process is well-suited for simultaneously densifying large quantities of porous articles and is particularly suited for making carbon/carbon brake disks. With appropriate processing conditions, a matrix with desirable physical properties can be deposited. However, the conventional CVI/CVD process is a very slow process and may require weeks of continual processing in order to achieve a useful density. Moreover, the surface tends to densify first resulting in xe2x80x9cseal-coatingxe2x80x9d that prevents further infiltration of reactant gas into inner regions of the porous structure and uniform deposition of the matrix material within the porous structure. Thus, this technique generally requires several surface machining operations that interrupt the densification process.
In a thermal gradient CVI/CVD process, a porous structure is heated so that a steep thermal gradient is generated which induces deposition in a desired portion of the porous structure. The thermal gradients may be induced by heating only one surface of a porous structure. For example, the thermal gradient may be generated by placing a porous structure surface against a susceptor wall in an electric induction heated furnace. The gradient created may be further enhanced by cooling an opposing surface, for example by placing the opposing surface of the porous structure against a liquid cooled wall. Deposition of the binding matrix progresses from the hot surface to the cold surface. The equipment for use in a thermal gradient process tends to be complex, expensive, and difficult to implement for densifying relatively large quantities of porous structures.
In a pressure gradient CVI/CVD process, the reactant gas is forced to flow through the porous structure by inducing a pressure gradient from one surface of the porous structure to an opposing surface of the porous structure. The flow rate of the reactant gas is greatly increased relative to the isothermal and thermal gradient processes which results in increased deposition rate of the binding matrix. This process is also known as xe2x80x9cforced-flowxe2x80x9d CVI/CVD. Conventional prior fixturing for pressure gradient CVI/CVD tends to be complex, expensive, and difficult to implement for densifying large quantities of porous structures. An example of a process that generates a longitudinal pressure gradient along the lengths of a bundle of unidirectional fibers is provided in S. Kamura, N. Takase, S. Kasuya, and E. Yasuda, Fracture Behaviour of C Fiber/CVD C Composite, Carbon ""80 (German Ceramic Society) (1980). An example of a process that develops a pure radial pressure gradient for densifying an annular porous wall is provided in U.S. Pat. Nos. 4,212,906 and 4,134,360. The annular porous wall disclosed by these patents may be formed from a multitude of stacked annular disks (for making brake disks) or as unitary tubular structure. For thick-walled structural composites, a pure radial pressure gradient process generates a very large, undesirable density gradient from the inside cylindrical surface to the outside cylindrical surface of the annular porous wall.
Finally, pulsed flow involves cyclically filling and evacuating a chamber containing the heated porous structure with the reactant gas. The cyclical action forces the reactant gas to infiltrate the porous structure and also forces removal of the cracked reactant gas by-products from the porous structure. The equipment to implement such a process is complex, expensive, and difficult to maintain. This process is very difficult to implement for densifying large numbers of porous structure.
The thermal gradient and pressure gradient processes have been combined by many workers resulting in a xe2x80x9cthermal gradient-forced flowxe2x80x9d process. Combining the processes appears to overcome some of the shortcoming of each of the individual processes and results in very rapid densification of porous structures. However, combining the processes also results in an increase in the complexity of the system since fixturing and equipment must be provided to induce both thermal and pressure gradients with some degree of control. A process for densifying small disks and tubes according to a thermal gradient-forced flow process is disclosed by U.S. Pat. No. 4,580,524; and by A. J. Caputo and W. J. Lackey, Fabrication of Fiber-Reinforced Ceramic Composites by Chemical Vapor Infiltration, Prepared by the OAK RIDGE NATIONAL LABORATORY for the U.S. DEPARTMENT OF ENERGY under Contract No. DE-AD05-840R21400 (1984). According to this process, a fibrous preform is disposed within a water-cooled jacket. The top of the preform is heated and a gas is forced to flow through the preform to the heated portion where it cracks and deposits a matrix. A process for depositing a matrix within a tubular porous structure is disclosed by U.S. Pat. No. 4,895,108. According to this process, the outer cylindrical surface of the tubular porous structure is heated and the inner cylindrical surface is cooled by a water jacket. The reactant gas is introduced to the inner cylindrical surface. Similar thermal gradient-forced flow CVI/CVD processes for forming various articles are disclosed by T. Hunh, C. V. Burkland, and B. Bustamante, Densification of a Thick Disk Preform with Silicon Carbide Matrix by a CVI Process, Ceram. Eng. Sci. Proc. 12[9-10] pp. 2005-2014 (1991); T. M. Besmann, R. A. Lowden, D. P. Stinton, and T. L. Starr, A Method for Rapid Chemical Vapor Infiltration of Ceramic Composites, Journal De Physique, Colloque C5, supplement nxc2x05, Tome 50 (1989); T. D. Gulden, J. L. Kaae, and K. P. Norton, Forced-Flow Thermal-Gradient Chemical Vapor Infiltration (CVI) of Ceramic Matrix Composites, Proc.-Electrochemical Society (1990), 90-12 (Proc. Int. Conf. Chem. Vap. Deposition, 11th, 1990) 546-52.
Fixturing and equipment for the thermal gradient forced flow CVI/CVD process can be complex as explained above since both thermal and pressure gradients must be induced with some degree of control. In order to obtain some degree of control in such devices, it is desirable to know the pressure within the furnace. In particular, it is desirable to be able to determine the pressure within a particular region of the furnace, and more specifically, the pressure near the porous structures themselves. Conventionally, the pressure of the reactant gas was generally measured before it entered the furnace. This pressure was then extrapolated to determine what the pressure in the furnace, near the porous structures, should be. This method is only accurate to within xc2x125-30%, due to the many unknowns, such as, but not limited to gas temperature and gas velocity through the preheater once the gas enters the furnace. Another way to measure pressure was to lower a pitot static tube into the furnace from a hole in the furnace lid. This method was very labor intensive and time consuming.
Therefore, a method to determine the pressure within the furnace during thermal-gradient forced flow CVI/CVD processes, as well as all other CVI/CVD processes, and apparatus for implementing that method are desired. Preferably this method should give constant, real time pressure readings. In particular, a method is desired to determine the pressure in the furnace very near the porous structures themselves, with enough accuracy to determine if there are pressure leaks before the reactant gas reaches the porous structures. Pressure leaks are not desirable because the deposition rate of the carbon onto the porous structures will be reduced. Furthermore, leaks cause poor gas flow distribution in the furnace and often times lead to sooting. Finally, apparatus for implementing the method is desirable.
According to an aspect of the invention, apparatus is disclosed for measuring pressure within a susceptor volume, within a CVI/CVD furnace during a CVI/CVD process wherein a reactant gas is introduced into the CVI/CVD furnace via a reactant gas line, and wherein the susceptor volume is bound by susceptor sides, a susceptor lid, and a floor plate, comprising: a pressure indicator tube having two ends, a first end and second end, wherein said first end is in the susceptor volume, and wherein the pressure indicator tube is located inside the reactant gas line; and a pressure indicator located outside of the CVI/CVD furnace, wherein said second end of the pressure indicator tube is connected to the pressure indicator in fluid communication therewith.